How to Choose a Good Magnesium Supplement: Reading the Label
Magnesium is one of the most popular supplements among people who train, often bought for cramps, sleep or recovery. Yet two tubs that both say "magnesium 500 mg" on the front can contain very different amounts of the mineral itself. Our editorial team explains how to read a magnesium label properly and what really separates a good product from an expensive disappointment.
Why athletes think about magnesium
Magnesium is involved in hundreds of enzyme reactions, including energy production, protein synthesis, muscle contraction and nerve signalling. It also contributes to bone structure and helps regulate the movement of calcium and potassium across cell membranes.
The UK Nutrient Reference Value (NRV) used on labels is 375 mg per day. National diet surveys suggest that a proportion of adults, particularly teenagers and young women, do not reach recommended intakes from food. Wholegrains, nuts, seeds, legumes and leafy greens are the richest sources.
Athletes lose some magnesium in sweat and urine, and those restricting food intake for weight categories are at higher risk of low intake. According to reviews by Nielsen and Lukaski, and by Zhang and colleagues, supplementation appears to benefit performance mainly in people who are deficient, with little effect when status is already adequate.
So the first question is not "which brand?" but "do I need it?". A food diary or a conversation with a dietitian is often more informative than a supplement purchase.
Elemental magnesium versus compound weight
Magnesium in supplements is always bound to another molecule, such as citrate, oxide or glycine. What matters for your body is the amount of elemental magnesium, the mineral itself. Some labels prominently state the weight of the whole compound, which can be several times higher.
For example, 1000 mg of magnesium citrate provides roughly 160 mg of elemental magnesium, while the same weight of magnesium oxide provides about 600 mg. Under UK labelling rules, the nutrition table must declare the amount of the nutrient (magnesium) per daily dose, so always check the table rather than the front of the pack.
The compound weight also explains why some forms come as large capsules or require several capsules per day. A product promising 400 mg of magnesium from glycinate will need around 2.8 g of compound, which is difficult to fit into one tablet.
If a label lists only "magnesium bisglycinate 1500 mg" and does not clearly state the elemental amount in the nutrition table, treat that as a warning sign about the manufacturer's transparency.
Choosing the form
Higher elemental content does not guarantee more magnesium absorbed. Magnesium oxide is dense in magnesium but relatively poorly soluble. Studies by Lindberg and colleagues, and later Walker and colleagues, found better absorption from magnesium citrate than from oxide, measured by urinary excretion.
| Form | Strengths | Drawbacks | Often chosen for |
|---|---|---|---|
| Citrate | Good solubility and absorption, inexpensive | Can loosen stools at higher doses | General use |
| Glycinate / bisglycinate | Usually gentle on the gut | Low elemental content, larger capsules | Sensitive stomachs, evening use |
| Oxide | High magnesium per tablet, cheap | Lower absorption, laxative effect | Budget products, constipation |
| Malate | Reasonable absorption | Less clinical research | General use |
| Chloride, lactate | Soluble | Taste in powders | Liquids, effervescent forms |
Claims that a particular form targets the brain, muscles or sleep should be treated cautiously. Magnesium L-threonate, for instance, has interesting animal data on brain magnesium, but human evidence is limited and it provides little elemental magnesium per dose.
For most people, citrate or glycinate offers a sensible balance. The best form is ultimately the one you tolerate and take consistently.
%NRV, upper limits and the small print
The %NRV figure shows how much of the 375 mg reference value one daily dose supplies. A product providing 100–200 mg per day (roughly 25–55% NRV) complements a reasonable diet. Higher amounts are not necessarily better.
The European Food Safety Authority, via the earlier Scientific Committee on Food opinion, set a tolerable upper level of 250 mg per day for supplemental magnesium, based on the risk of diarrhoea. In the UK, the Expert Group on Vitamins and Minerals published a guidance level of 400 mg per day of supplemental magnesium. These limits do not apply to magnesium naturally present in food.
Check the "other ingredients" line too. Fillers, anti-caking agents and capsule shells are normal, but some powders contain sweeteners, flavourings or added sugar. Effervescent tablets may contribute meaningful amounts of sodium, relevant for people watching blood pressure.
Look at the stated daily dose and the number of servings per container. A cheaper tub may require four capsules a day, making it more expensive per milligram of elemental magnesium than a pricier competitor.
A quick label checklist
- Elemental magnesium per daily dose is shown in the nutrition table.
- The form is named precisely (e.g. "magnesium citrate", not "magnesium complex").
- The daily dose stays within sensible limits for supplemental magnesium.
- Additives are minimal and appropriate for your needs.
- Health claims match those authorised in Great Britain, such as "contributes to normal muscle function" or "contributes to a reduction of tiredness and fatigue".
- For tested athletes, the product carries an independent batch-testing certification.
Be wary of claims to cure cramps, anxiety or insomnia. These are not authorised health claims, and they suggest a company prepared to stretch the rules elsewhere too.
People with kidney disease should not take magnesium supplements without medical advice, because impaired kidneys cannot excrete excess magnesium efficiently. Magnesium can also reduce absorption of some antibiotics and osteoporosis drugs, so separate doses as advised by a pharmacist.
Finally, store the product in a dry place and check the best-before date. Powders in particular can clump if exposed to humidity.
Conclusions of the editorial team
When choosing magnesium, look past the front of the pack. The elemental amount in the nutrition table, the named form and the daily dose tell you far more than marketing slogans.
Citrate and glycinate suit most people; oxide is cheaper but less well absorbed and more likely to cause digestive upset. Stay within established upper levels for supplemental magnesium.
Supplements are most useful when dietary intake is low. Improving food choices remains the foundation.
See also our articles on zinc and magnesium combinations, electrolytes for endurance training, and how to spot misleading supplement claims.
References
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for magnesium. EFSA J. 2015;13(7):4186.
- Expert Group on Vitamins and Minerals. Safe Upper Levels for Vitamins and Minerals. London: Food Standards Agency; 2003.
- Lindberg JS, Zobitz MM, Poindexter JR, Pak CY. Magnesium bioavailability from magnesium citrate and magnesium oxide. J Am Coll Nutr. 1990;9(1):48–55.
- Walker AF, Marakis G, Christie S, Byng M. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res. 2003;16(3):183–191.
- Nielsen FH, Lukaski HC. Update on the relationship between magnesium and exercise. Magnes Res. 2006;19(3):180–189.
- Zhang Y, Xun P, Wang R, Mao L, He K. Can magnesium enhance exercise performance? Nutrients. 2017;9(9):946.
